> deteriorate productivity in ways that will be difficult to understand
Exactly! This can be very difficult to quantify; as this kind of debt is undetectable by standard gates (linter, static analysis, etc.). Instead, it is more nefarious... introducing coupling, where none was needed, or long-winded tests; that test very little, and make the code hard to refactor. I don't have a measure for this beyond my intuition of working with code for decades.
What makes it all worse, is when the features are 'delivered' and marked as 'complete' by these folks, they are difficult to take back! Especially if their delivery was identified as a key objective to a quarter.